The effect of SO2 upon the high temperatureoxidation of Cu

The oxidation of Cu in the temperature range 700–1000°C has been investigated in a wide range of A + O₂ + SO₂ environments using an automatic recording balance to study reaction kinetics. The structure and composition of the scales were investigated by optical microscopy, X-ray diffraction, electron-probe and X-ray fluorescence analysis and infra-red spectrophotometry.

The observed changes in reaction kinetics and scale structure have been interpreted in terms ofenhanced cation transport through the scale constituents brought about by dissolution of small concentrations of S in the oxide scales.

Under certain conditions the oxidation mechanism was further modified by intergranular penetration of the CuO layer by a low melting point eutectic formed between CuO and the basic sulphate CuOCuSO₄. Under these conditions the reaction rate increased by several orders of magnitude compared with the rates in O₂ alone.

Résumé

L'oxydation du Cu dans la bande de températures de 700 à 1000°C a été étudiée dans unegamme étendue de milieux A + O₂ + SO₂ à l'aide d'une balance enregistreuse automatique pour étudier la cinétique de la réaction. La structure et la composition des écailles ont été étudiées par microscopie optique, par diffraction de rayon X, par sonde électronique, analyse de la fluorescence de rayons X, et par spectrophotométrie des infrarouges.

Les changements de cinétique de réaction et de structure d'écailles observés ont été interprêtés enfonction de transport de cations améliorés à travers les composants de l'écaille survenus à la suite de la dissolution de faibles concentrations de S dans les écailles d'oxyde.

Dans certaines conditions, le mécanisme d'oxydation a été encore modifié par la pénétrationintergranulaire de la couche de CuO par un eutectique à point de fusion bas formé entre le CuO et le sulfate de base CuOCuSO₄. Sous ces conditions le taux de réaction s'est accru de plusieurs ordres de grandeur en comparaison avec les taux dans le O₂ seul.

Zusammenfassung

Die Oxidation von Cu wurde zwischen 700 and 1000°C in einer A/O₂/SO₂2-Atmosphäre über einen breiten Konzentrationsbereich mit Hilfe einer automatischen Registrierwaage untersucht, um Aufschluß über die Reaktionskinetik zu erhalten. Die Struktur and die Zusammensetzung der Folien wurden durch lichtmikroskopische Untersuchungen, Röntgenbeugungs-Messungen, Mikrosonden- und Röntgenfluoreszenz-Analyse, sowie durch Infrarot-Spektrophotometrie untersucht.

Die beobachteten Änderungen der Reaktionskinetik und der Struktur der Schichten wurden als Zunahme des Kationen-Transportes durch die Bestandteile der Schicht, hervorgerufen durch die geringe S-Löslichkeit in der Oxidschicht, interpretiert. Unter bestimmten Bedingungen wurde der Oxidationsmechanismus weiterhin durch das Eindringen eines niedrigschmelzenden Eutektikums aus CuO and dem basischen Sulfat CuO - CuSO₄ zwischen die Korngrenzen der CuO-Schicht verändert. Unter diesen Bedingungen steigt die Reaktionsgeschwindigkeit um mehrere Größenordnungen verglichen mit der Oxidation in reinem O₂ an.     

Influence of epitaxial stress on spinodal decomposition in binary alloys with large size effects: application to the Au–Ni system

Within the framework of Cahn’s pioneering theory of spinodal decomposition, the effect of the epitaxial stress on the stability of a binary alloy solid solution against infinitesimal compositional fluctuations is revisited. It is shown that the elastic energy term due to a coherent lattice mismatch with a substrate modifies the linearised diffusion equation. This term is particularly high when size effects between the two constituents are strong. It is then necessary to take into account anharmonic terms in the expression of the elastic energy. From the modified diffusion equation, a new spinodal curve, called the epitaxial coherent spinodal, may be calculated for different substrate lattice parameters. The results of the model are compared with recent experiments on coherently grown (001) Au–Ni alloys. A modulated structure has been evidenced upon annealing which is in good agreement with our calculations.

Résumé

Dans le cadre de la théorie de la décomposition spinodale développèe par Cahn, nous présentons l’effet des contraintes épitaxiales sur la stabilitéd’une solution solide d’alliage binaire soumise ádes fluctuations de compostion infinitésimales. Les contraintes induites par l’épitaxie cohérente sur un substrat ayant un paramètre de maille différent de celui de la solution solide considérée sont àl’origine d’un terme d’énergie élastique dont il faut tenir compte dans l’équation de diffusion. Ce terme devient particulièrement important lorsque les effets de taille entre les deux constituants sont grands. Nous montrons qu’il est alors nécessaire de considérer dans l’expression de l’énergie élastique les termes anharmoniques. Une nouvelle spinodale, appelée spinodale cohérente épitaxiale, peut être calculée pour différentes valeurs du paramètre de maille du substrat. Les calculs sont comparés avec les résulats expérimentaux obtenus sur des alliages Au–Ni (001) élaborés de manière cohérente. Une structure modulée a étémise en evidence au cours de recuits, ce qui est en accord avec les résultats du modèle.     

Solution chemistry within stress-corrosion cracks in alloy steels

The acidity and the concentrations of metal ions in the solution within progressing stress-corrosion(s.c.) cracks in some alloy steels were estimated. It was found that the solution is always pH 3·6–3·8 near the crack tip regardless of the particular type of alloy steel. In quenched-and-tempered alloy carbon steels the metallic elements are found in the solution in about the same proportions as in the steel. The relatively constant acidity produced during s.c.c. of the alloy steels is discussed in terms of the hydrolysis of Fe ions.

Résumé

Une estimation a été faite de l'acidité et des concentrations d'ions métalliques dans lasolution dans des fêlures de corrosion par contrainte (s.c.) en progression dans certains alliages d'acier. Il a été trouvé que la solution est toujours à un pH de 3,6 à 3,8 prés de la crête de la fêlure quelque soit le type particulier d'alliage d'acier. Dans les alliages d'acier au carbone trempés les éléments métalliques sont rencontrés dans la solution dans les mêmes proportions environ que dans l'acier. L'acidité relativement constante produite durant le fendillement s.c.c. des alliages d'acier est discutée en fonction de l'hydrolyse des ions de Fe.

Zusammenfassung

Der Säuregrad und die Konzentrationen von Metallionen in der Lösung innerhalbfortschreitenden Spannungskorrosion (s.c.) Rissen werden für einige Legierstahle geschätzt. Es wurde gefunden, dass die Lösung immer nahe der Spitze des Risses pH 3,6–3,8 ohne Rücksicht auf die besondere Art des Legierstahls ist. In abgeschreckten und getemperten, legierten Flusstahlen werden die metallischen Elemente in etwa den gleichen Verhältnissen wie beim Stahl in der Lösung gefunden. Der relativ konstante Säuregehalt, der während s.c.c. des Legierstahls entsteht, wird im Sinne von Hydrolyse der Fe Ionen behandelt.     

Investigation of anodic passivity of Ni

The anodic passivity of Ni in 1N H₂SO₄ has been investigated by impedance measurementsof the system passive Ni-electrolyte solution. Using the bridge method the impedance was experimentally determined as a series combination of R and C components at frequencies from 20–31,000 Hz. From these results the complex nature of the passive layer may be concluded. We tried to apply the Scholte-van Geel and Young's analysis, but many difficulties arose. It is interesting to note that, by the formal application of these methods, even negative elements of resistance (R_i) were obtained by means of computer calculations. The analysis of the complex dielectric constant was also performed. The analysis points to the presence of relaxation phenomena and enables the understanding of results obtained experimentally.

Résumé

La couche anodique passive sur le Ni dans 1N H₂SO₄ s'analyse par la methode del'impédance. Par la méthode du pont. l'impédance est expérimentalement déterminée en tant que combinaison sérielle des componentes R et C dans la dépendance d'une fréquence de 20 Hz–31,000 Hz. D'après ces résultats, on conclut quant a la nature complexe de la couche passive. Nous avons essayé d'appliquer l'analyse de Scholte-van Geel-Young. A cette occasion, nous avons rencontré de grandes difficultés. II est intéressant de noter que, dans l'application formelle de ces analyses, nous avons même obtenu, par le calcul de l'ordinateur, des éléments négatifs de résistance (R_i). A également été effectuée l'analyse de la constante diélectrique complexe. Cette analyse révèle la présence de phénomenes relaxatifs et permet de comprendre les résultats experimentaux obtenus.

Zusammenfassung

Die anodische Passivschicht wurde an Ni in 1N H₂SO₄ mittels der Impedanz-Methode untersucht. Die Brückenmethode bestimmte experimental die Impedanz als eine Serien-Kombination der R and C Komponenten im Frequenzbereich von 20 Hz bis 31.000 Hz. Diese Resultate lassen auf die komplexe Natur der Passivschicht schliessen. Es wurde auch die Anwendung der Scholte von Geel- bzw. der Young-Analyse versucht. Wir trafen aber hier an wesentliche Schwierigkeiten. Es ist bemerkenswert, dass wir in der formälen Anwendung dieser Analysen mittels Kompjuter-Berechnungen auch negative Widerstand-elemente (R_i) bekamen. Es wurde auch die Analyse der komplexen dielektrischen Konstante durchgeführt. Diese Analyse weist auf die Anwesenheit der Relaxationsphenomena unf ermöglicht damit das Verständnis der erlangten Ergebnisse.     

Sur de nouveaux matériaux à base du bismuth trivalent de structures fluorine et pyrochlore

The study of the pseudo-binary systems Bi₂O₃-A₂B₂O₇, with A Cd²⁺, Pb²⁺, Mg²⁺ and B & Nb⁵⁺, Ta⁵⁺, reveals two types of solid solutions. The first one with Bi_1−2xCd_xB_xO_2−y (y = (1 − x)/2) formulation shows a fluorite-type structure whereas the second type with A_2−zB_2−zBi_2xO_7−z/2 formulation is obtained for A₂B₂O₇-rich compositions and possesses structures deriving from the pyrochlore type. The composition dependence of the unit cell parameters has been correlated to possible substitution mechanisms.

Résumé

D'une manière générale, l'étude des pseudo-binaires Bi₂O₃---A₂B₂O₇ (avec A Cd²⁺, Pb²⁺, Mg²⁺ et B Nb⁵⁺, Ta⁵⁺) a permis de mettre en évidence deux types de solutions solides: le premier de structure fluorine déficitaire en anions a pour formule Bi_1−2xCd_xB_xO_2−y (y = (1−x)/2); la seconde solution solide de composition A_2−z B_2−z Bi_2zO_7−z/2 est obtenue pour des compositions riches en A₂B₂O₇ et possède une structure qui dérive de celle du pyrochlore. Les paramètres cristallins des diverses phases isolées ont été déterminés à 298 K à partir des diffractogrammes X. Leur évolution a été corrélée avec le taux de substitution.     

Le système binaire Pd---Si

The Pd---Si phase diagram has been revised in details (178 alloys) on carefully annealed specimens by microprobe analysis, X-rays diffraction and DTA. The Pd₂Si intermetallic phase (m.p. 1404 °C) is confirmed to be stoichiometric with the C22-structure, but within the ranges of composition 33.3–33.9 and 34.5 at.% Si it diversifies in three very similar crystalline structures formed peritectically at 1053–1083 and 1072 ± 2 °C respectively. PdSi only exists in a very short range of temperature (888–908 °C melting congruently), whereas Pd₅Si (m.p. 856 °C) disappears below 811 °C, likewise producing four very narrow and similar phases at 16 at.% Si (below 811 °C), 17.7 (below 795 °C), 18.2 at.% Si (between 819 and 764 °C) and 21.0 at.% Si (between 792 and 753 °C), all these phases being formed by peritectoïd transformations. The only Pd₃Si-phase (congruent m.p. 1074 °C) remains stable at low temperature.

Résumé

L'étude très détaillée du diagramme de phase Pd---Si (178 alliages) révèle que la phase intermétallique Pd₂Si (Fus. 1404 °C) est stoechiométrique avec une structure C22. Cependant, elle se diversifie en trois structures cristallines ordonnées étroites, qui se forment péritectiquement à 1053–1083 et 1072 ± 2 °C, dans un domaine restreint de composition 33,3–33,9 et 34,5 at.% Si respectivement. Le composé PdSi (Fus. congruente 908 °C) existe dans un très faible domaine de température (888–908 °C). Pd₅Si (Fus. 856 °C) disparaît au-dessous de 811 °C en donnant naissance à quatre phases très proches et très similaires à 16 at.% Si (au-dessous de 811 °C), 17,7 (au-dessous de 795 °C), 18,2 at.% Si (Pd₉Si₂) (entre 819 et 764 °C) et 21,0 at.% Si (entre 792 et 753 °C), toutes ces phases sont issues de transformations péritectoïdes. Seule la phase Pd₃Si se maintient sans autre modification à basse température (Fus. congruente 1074 °C).     

Influence of titanium nitride addition on the microstructure and mechanical properties of TiC-based cermets

In this paper, the TiC-based cermets with addition of TiN were fabricated by a conventional powder metallurgy process. The titanium nitride (TiN) and titanium carbide (TiC) used as starting powders have been synthesized by the self-propagating high temperature synthesis (SHS) method. This exothermic reaction, easy to process, allows to obtain fine and original powders from low-cost raw materials. Cermets obtained by sintering powders of TiC and Mo₂C with nickel binder phase are investigated. The effect of TiN adding on the microstructure and the mechanical properties of these composites are studied. Microstructures have been observed by scanning electron microscopy (SEM). Room temperature mechanical properties such as Young’s modulus, fracture toughness and microhardness have been measured and related to morphology and chemical composition of the samples. Tribological experiments were also performed and the friction coefficient of a cermet containing titanium nitride was compared with that of other hard materials. The SHS starting powders used present some particularities, as it was shown in a previous study [Matériaux à base de carbures et nitrures, pour coupe et usure, obtenus à partir de poudre SHS, PhD Thesis, INSA Lyon, 2004]. The purpose of this work is to show that results concerning the impact of TiN addition on microstructure and mechanical properties obtained on bulk specimens, from these original starting powders, are similar to the ones obtained on alloys from commercial starting powders.     

Microstructure and Mechanical Properties of Graphene Reinforced K418 Superalloy by Selective Laser Melting

In order to obtain the superalloy with excellent properties, graphene reinforced K418 nickel base superalloy (GNPs/K418 composite) was prepared by selective laser melting technique in this study. Through systematically comparing and analyzing the microstructure and mechanical property of K418 superalloy and GNPs/K418 composite, it is found that the percentage of small-diameter grain (≤ 15 μm) increased from 84% to 90%, and the max strength of grain orientation (<001>) reduce from 5.76 to 4.17 due to the addition of GNPs. And GNPs can also improve the height and the full width at the half peak of the strong diffraction peak of GNPs/K418 composite. Besides, GNPs/K418 composite is a kind of sandwiched structure, which is consist of GNPs, carbides, and K418 matrix. Therefore, the hardness of the GNPs/K418 composite is 4.1% and 6.9% higher than that of the K418 matrix in the transverse and vertical direction, respectively. The room temperature tensile strength of the GNPs/K418 composite is 9% higher than that of the K418 matrix. And the 600 °C and 900 °C tensile strengths of the GNPs/K418 composite are 7.6% and 10.4% higher than that of the K418 matrix, respectively. It is inferred that the effect of graphene on K418 matrix strengthening is mainly fine grain strengthening and Orowan strengthening. However, the elongation rate of the composite material is reduced, which is attributed to crack sprouting at the interface between the matrix and GNPs under high stress.     

Precipitation behavior and its effects on tensile properties of FeCoNiCr high-entropy alloys

In this work, we present a systematic study on the precipitation behavior and mechanical properties of a FeCoNiCr-based high-entropy alloy alloyed with dilute amounts of Ti and Al, (FeCoNiCr)_100-x-yTi_xAl_y (where x = 1–3, y = 4–9 at.%). It was found that, upon aging, nano-sized L1₂-Ni₃(Ti, Al) particles are formed within grains, whilst L2₁-(Ni, Co)₂TiAl Heusler particles are formed mainly along grain boundaries. The relative thermal stability of the two phases were studied at different aging temperatures (700–900 °C) with various durations of time (up to 48 h) and the results were directly compared with Thermo-calc calculations. Tensile tests were also conducted on alloys aged under different conditions. The measured properties, including strength and ductility, were correlated with the microstructure of aged (FeCoNiCr)_100-x-yTi_xAl_y alloys, with particular attention on the distribution and morphology of the two kinds of precipitate. Whereas both phases could contribute to the strengthening of the alloys via either Orowan bowing or particle shearing mechanism, the brittle (Ni, Co)₂TiAl Heusler phase was found to mainly affect the tensile plasticity. A simple composite model was proposed to describe the plastic strain of alloys. Based on observed microstructure and its corresponding mechanical performance, the alloy with the composition of (FeCoNiCr)₉₄Ti₂Al₄, when aged between 700 and 800 °C, gives the best balanced strength/ductility properties.     

Quantitative representation of the anisotropy and tension/compression asymmetry of the critical resolved shear stress of an L12-ordered intermetallic

The anomalous anisotropy and tension (t)/compression (c)-asymmetry of the critical resolved shear stress τ_o of the L1₂-long-range ordered intermetallic γ′_NIM are analysed in the light of a published (Miner RV, Gabb TP, Gayda J, Hember, KJ. Met Trans A 1986; 17A: 507) analytical expression

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. Particles of γ′_NIM strengthen the commercial nickel-base superalloy NIMONIC 105. characterises the orientation of the specimen, and T_o≈300 K and T_peak≈ 1000 K mark the temperature (T) range in which τ_o is anomalous. Except for γ′_NIM-single crystals with the [011]-orientation, the formula describes the anisotropy and (t/c)-asymmetry of the critical resolved shear stress satisfactorily.     

Structural and magnetic characterization of martensitic Ni–Mn–Ga thin films deposited on Mo foil

Three martensitic Ni_51.4Mn_28.3Ga_20.3 thin films sputter-deposited on a Mo foil were investigated with regard to their crystal and magnetic domain structures, as well as their magnetic and magnetostrain properties. The film thicknesses, d, were 0.1, 0.4 and 1.0μm. X-ray and electron diffraction patterns revealed a tetragonal modulated martensitic phase (10 M) in the films. The surface topography and micromagnetic structure were studied by scanning probe microscopy. A maze magnetic domain structure featuring a large out-of-plane magnetization component was found in all films. The domain width, δ, depends on the film thickness as . The thickness dependencies of the saturation magnetization, saturation magnetic field and magnetic anisotropy were clarified. Beam cantilever tests on the Ni–Mn–Ga/Mo composite as a function of magnetic field showed reversible strains, which are larger than ordinary magnetostriction.     

Corrosion behavior of carbon steel in chloride and bicarbonate ion-enriched and CO2-saturated solutions

This article addresses the knowledge gap in the area of the synergistic effect between Cl⁻ and by comprehensively evaluating the corrosion behavior of AISI 1020 carbon steel exposed to the CO₂-saturated solutions containing Cl⁻ and/or over the time period of 6–72 hrs at 60°C. The results show a synergistic mechanism between Cl⁻ and in the pitting corrosion of carbon steel. accelerates the localized precipitation of FeCO₃ crystals by means of increasing pH and adsorption, which is a prerequisite for pitting initiation. Cl⁻ contributes to the catalytic dissolution of the Fe matrix along with film breakdown via the local accumulation of oxygen vacancies, and the pitting corrosion of carbon steel is, thus, further exacerbated.